The purpose of this study is to evaluate a method for early detection of Necrotizing Enterocolitis (NEC). This a bowel disease effecting infants with unknown cause, but is believed to be caused by a combination of bacterial infection and low blood flow of the intestines. If severe, it can cause necrosis (death of part the bowel) perforation, infection and death. Currently there is no known method for early detection of NEC. In this cooperative study, researchers at Stanford and University of California at San Francisco are trying to see if early NEC can be detected without the need to draw blood or other invasive procedures. Infants are given a small amount of Iohexol and have their urine collected for the next 12 hours and examined for Iohexol levels. In a preliminary study infants with NEC, showed that had increased levels of Iohexol in their urine. We hope that this study will lead to a method that doctors can use to diagnose NEC early and start life saving therapy sooner than currently is possible.